In Internet Protocol (IP) Multimedia Subsystem (IMS), the clients have to register to the network prior to receiving a service. The Home Subscriber Server (HSS) node is a critical node in the network and if the HSS is unavailable to process signaling requests, then the IMS network is unable to deliver services to the clients.
In Voice over Long Term Evolution (VoLTE) networks, there are many queries to the HSS during an attach/registration process. If the HSS is unavailable, this may lead to a critical situation where users in the network, all trying to register for the service, generate mass signaling.
According to the 3GPP standards, if the Interrogating Call Session Control Function (I-CSCF) is unable to complete the user registration status query procedure User-Authorization Request (UAR) to the HSS, it shall respond back to the UE with a negative User-Authorization Answer (UAA) response; either 403 Forbidden or 480 Temporary Unavailable.
However if the HSS is in an overload condition or if there is a failure in the communication link between the HSS and the I-CSCF, there could be thousands, if not millions, of users out of service in the IMS network. When these users are unable to register or reregister, they retry a number of times. The retry signaling creates extra signaling on all the network nodes in the registration process including the P-CSCF and I-CSCF in addition to the HSS.
In some cases, the UEs, after a number of retries, attempt to reregister in a secondary site. In this secondary site, communications channels towards the HSS may be available but this can lead to yet more signaling load on the HSS.
In a VoLTE network, both the Evolved Packet Core (EPC) network and the IMS network signal towards the HSS and this leads to a combined total of up to 16 signaling requests towards the HSS.
In a Voice over WI-FI (Vo WI-FI) network, WI-FI being a technology for wireless local area networking with devices based on the IEEE 802.11 standards, whenever the UE changes access networks between LTE and WI-FI, the UE performs a reregistration process.
The previous examples show that the reregistration process is a frequent process and is a signaling intensive process. There is currently no graceful mechanism to handle a scenario where the HSS is unreachable and the registration process is able to be successfully completed.